This invention relates to numerically controlled machine tool systems, and more particularly, to numerically controlled machine tool systems having programmable tool offset.
Many machine tools, such as grinding machines and milling machines, include a workpiece positioning table and a rotating cutting element, with the cutting element having a cutting surface laterally disposed about the rotational axis of the cutting element. In operation, a workpiece is affixed to the positioning table. The machine tool controls the relative motion of the cutting element rotational axis with respect to the workpiece so that the cutting element is offset from a desired workpiece by the cutting element radius, establishing a contact point between the cutting surface and the desired contour. The motion of the cutting element is typically controlled so that the contact point lies in a plane perpendicular to the rotational axis of the cutting element. As the cutting element wears, its decrease in radius must be accommodated in order to maintain a contact point with the desired contour.
In addition to the effect of cutting element wear, various cutting elements exhibit differing cutting efficiencies, depending upon the cutting element material, the workpiece material, or the dynamics (such as direction-related efficiencies) of the cutting element driving servos. As a result of all of these factors, the establishing of control of the cutting element axis to achieve a desired workpiece contour is a complex problem.
A numerically controlled machine tool operator, or programmer, may work from a drawing of a part to be machined which defines edge points on a workpiece. Typically, the operator must generate data signals for the machine tool which define a desired path for the cutting element with respect to the workpiece so that the cutting surface of the cutting element includes a contact point in common with the desired workpiece contour at each of the points defined in the drawing, or other specification of the workpiece.
In one prior art approach to this programming procedure, the operator may calculate the location of points with respect to the workpiece which are offset from the desired workpiece contour by the current radius of the cutting element. By generating an appropriate number of these desired cutting element path points, and controlling the cutting element axis to follow that path, the workpiece may be machined to the desired contour. This approach requires substantial operator effort to work from the drawing and from the known cutting element radius to determine the precise points for that cutting element path.
In an alternative approach, a numerically controlled machine tool system may require the operator only to program in the coordinate values representative of selected points on the desired contour of the workpiece, together with a desired offset which the cutting element is to be displaced in a predetermined direction from the path defined by the programmed desired contour points. This approach is, of course, a much simpler task for the operator to perform compared with the previously mentioned approach wherein the operator must compute the actual offset path for the cutting element. In the latter approach, the machine tool system includes a computing apparatus which performs the necessary calculations to generate appropriate signals for directing the cutting element along the offset path. However, in this latter approach, the prior art systems are suitable only for providing a single offset value for complete programming machine tool operation. While this approach is effective for relatively short machining sequences and for sequences wherein the machine cutting is achieved with equal facility in different directions, there are substantial disadvantages in applications where it is desired to accomplish a series of different machining operations having different offset values.
In addition, many conventional machine tool systems are characterized by different efficiencies in different directions, for example, cutting along a first axis may be performed with one degree of efficiency while cutting along a second axis perpendicular to the first axis may be characterized by a somewhat different efficiency. In such cases, the prior art systems requiring a single offset value to be programmed for a set of operations are not suitable for these applications.
In some applications of machine tool systems, it is required to provide a milling or grinding operation along portions of a workpiece contour which may be characterized as locally convex, i.e. where interconnecting straight line segments for three successive points on the workpiece contour are defined by an angle exterior to the workpiece which is greater than 180 degrees. In some applications, the desired contour connecting three successive points A.sub.1, A.sub.2 and A.sub.3 defining a locally convex contour is characterized by a piecewise continuous, or step, first derivative at point A.sub.2 (for example, where the contour for A.sub.1, A.sub.2 and A.sub.3 is piecewise linear, or where the contour is formed by two circular arcs of differing radius and intersecting at A.sub.2, or where the contour is formed by a straight line segment and a circular arc intersecting at A.sub.2). Using the conventional approach to perform such operations for points A.sub.1, A.sub.2 and A.sub.3, the cutting element is controlled with respect to the workpiece so that the cutting element axis is directed along a first offset path substantially parallel to a first line segment joining points A.sub.1 and A.sub.2 (and which is offset from that line segment by the cutting element radius) to an intermediate point along that first path which is sufficiently beyond the point A.sub.2 so that the cutting element axis may then be controlled to pass along a second offset path substantially parallel to a second line segment joining points A.sub.2 and A.sub.3 (and which is offset from that second line segment by the cutting element radius). With this approach, the line segments and corresponding offset paths may be either straight or curved.
In these cases, as the cutting element axis approaches the neighborhood of the intermediate point (corresponding to the intersection of the first and second offset paths), the cutting surface is separated from the desired workpiece contour (as defined by points A.sub.1, A.sub.2 and A.sub.3) and thus the cutting element does not at all times maintain a contact point with the desired workpiece contour. As a result, in these applications, there is an inefficiency in time utilization in requiring the cutting element to travel while not maintaining a contact point with the workpiece.
A further disadvantage to this prior art technique arises when the cutting element is required to machine a workpiece at two points simultaneously, for example, when grinding a slot or groove. Where the desired slot or groove has a locally convex boundary that follows a curve having a piecewise continuous first derivative at one or more points, the above-noted approach requires that the cutting element cut a substantial amount of excess material from the side of the groove opposite to the locally convex portion while the cutting element is in the neighborhood of the intermediate point along the offset path. An alternative prior art approach to machining such a slot or groove is to program a circular motion for the cutting element axis at the locally convex portions. While this latter approach does reduce the requirement for the excess material cutting, the resultant groove has a smoothed contour at the step derivative points, rather than a sharp edge which may be achieved by the first noted approach of overshooting and then returning along a second line segment.
Accordingly, it is an object of the present invention to provide a numerically controlled machine tool system having a tool offset capability wherein selected points on a desired workpiece contour may be programmed for individually tailored offset characteristics.
A further object is to provide a numerically controlled machine tool system which may automatically accommodate programmed machining operations, with each machining operation having a characteristic tool offset.
Still another object is to provide a numerically controlled machine tool system which may accommodate sharp corner machining operations with minimum material cutting requirements.